Distributed denial of service (DDOS) attacks occur when a malicious user seeks to stop or severely limit access to a website. To achieve this, the malicious user may attack the website in a variety of ways, such as attempting to overwhelm the servers of the website by bombarding the servers with requests.
Currently, most servers implement in some fashion, an intrusion detection system (IDS) which is used to help determine when a DDOS attack is occurring, and where the DDOS attack is coming from. Typically, an IDS sounds alarms and alerts human operators that something is wrong. It is then up to the human operators to determine how to respond to the DDOS attack.